RISC-V: Make VXRM as global register [PR118103]

Inspired by PR118103, the VXRM register should be treated almost the
same as the FRM register, aka cooperatively-managed global register.
Thus, add the VXRM to global_regs to avoid the elimination by the
late-combine pass.

For example as below code:

  21   │
  22   │ void compute ()
  23   │ {
  24   │   size_t vl = __riscv_vsetvl_e16m1 (N);
  25   │   vuint16m1_t va = __riscv_vle16_v_u16m1 (a, vl);
  26   │   vuint16m1_t vb = __riscv_vle16_v_u16m1 (b, vl);
  27   │   vuint16m1_t vc = __riscv_vaaddu_vv_u16m1 (va, vb, __RISCV_VXRM_RDN, vl);
  28   │
  29   │   __riscv_vse16_v_u16m1 (c, vc, vl);
  30   │ }
  31   │
  32   │ int main ()
  33   │ {
  34   │   initialize ();
  35   │   compute();
  36   │
  37   │   return 0;
  38   │ }

After compile with -march=rv64gcv -O3, we will have:

  30   │ compute:
  31   │     csrwi   vxrm,2
  32   │     lui a3,%hi(a)
  33   │     lui a4,%hi(b)
  34   │     addi    a4,a4,%lo(b)
  35   │     vsetivli    zero,4,e16,m1,ta,ma
  36   │     addi    a3,a3,%lo(a)
  37   │     vle16.v v2,0(a4)
  38   │     vle16.v v1,0(a3)
  39   │     lui a4,%hi(c)
  40   │     addi    a4,a4,%lo(c)
  41   │     vaaddu.vv   v1,v1,v2
  42   │     vse16.v v1,0(a4)
  43   │     ret
  44   │     .size   compute, .-compute
  45   │     .section    .text.startup,"ax",@progbits
  46   │     .align  1
  47   │     .globl  main
  48   │     .type   main, @function
  49   │ main:
       |     // csrwi   vxrm,2 deleted after inline
  50   │     addi    sp,sp,-16
  51   │     sd  ra,8(sp)
  52   │     call    initialize
  53   │     lui a3,%hi(a)
  54   │     lui a4,%hi(b)
  55   │     vsetivli    zero,4,e16,m1,ta,ma
  56   │     addi    a4,a4,%lo(b)
  57   │     addi    a3,a3,%lo(a)
  58   │     vle16.v v2,0(a4)
  59   │     vle16.v v1,0(a3)
  60   │     lui a4,%hi(c)
  61   │     addi    a4,a4,%lo(c)
  62   │     li  a0,0
  63   │     vaaddu.vv   v1,v1,v2

The below test suites are passed for this patch.
* The rv64gcv fully regression test.

	PR target/118103

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_conditional_register_usage): Add
	the VXRM as the global_regs.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/pr118103-2.c: New test.
	* gcc.target/riscv/rvv/base/pr118103-run-2.c: New test.

Signed-off-by: Pan Li <pan2.li@intel.com>